1. Field of the Invention
The present invention relates to a photosensitive composition and a planographic printing plate precursor, in particular, to an infrared laser-compatible photosensitive composition suitable for use in so-called the direct-type planographic printing plate precursors, which allow direct plate-making from digital signals from computer and the like, and a planographic printing plate precursor employing the photosensitive composition.
2. Description of the Related Art
In the recent rapid progress in laser technology, smaller and higher-performance solid state and semiconductor lasers having an emission wavelength in the range from the near-infrared to infrared region are becoming readily accessible. For that reason, these lasers are very useful as the exposure-light sources for direct plate-making employing the digital data from computers and the like.
Infrared laser-compatible negative planographic printing plate precursors that utilize infrared lasers having an emission wavelength in the infrared region as the exposure-light source have a negative recording layer made of a photosensitive composition containing an infrared absorbing agent, radical generator, and polymerizable compound. Image portions on the negative recording layer are formed by the recording method wherein the exposed portions of recording layer are hardened by polymerization triggered by the radicals generated by light or heat as the initiator. Alternatively, when the infrared absorbing agent is a dye, the image portions are formed by the recording method wherein the exposed portions of recording layer are hardened by polymerization triggered by the radicals generated by electron transfer from the infrared ray-absorbed dye as the initiator.
In addition, photosensitive compositions and planographic printing plates employing a combination of a resol resin, novolak resin, infrared absorbing agent, and photochemical acid generator are disclosed in Japanese Patent Application Laid-Open (JP-A) Nos. 7-20629 and 7-271029, while planographic printing plates employing a combination of a particular polymer, photochemical acid generator, and near-infrared sensitizing dye are disclosed in JP-A Nos. 11-212252 and 11-231535. Hardened portions (image portions) on these photosensitive compositions and planographic printing plates disclosed are formed by the mechanism that the curing reaction is triggered by the acids generated by a photochemical acid generator as the initiator.
However, it is difficult to provide such photosensitive compositions wherein the curing reaction for image formation is triggered by the photopolymerization initiator or photochemical acid generator described above with sufficiently high photosensitivity in the near-infrared region. The photosensitivity is insufficient for applying these compositions to scanning exposure using infrared laser beam.
The curing reaction employing a photopolymerization initiator or photochemical acid generator often does not proceed satisfactorily only by exposure. Accordingly, when such a polymerizable composition is applied onto a planographic printing plate precursor, a heating treatment was indispensable for plate-making, as it is required after exposure or development for accelerating and completing the curing reaction. However, the heating treatment not only lowers the production efficiency but also contains a factor that may lead to deterioration in quality. For example, it is difficult to keep the difference in solubility between the exposed and unexposed portions on a certain level, and thus the exposed portions are also dissolved in the developing solution when the heating is insufficient, while the unexposed portions are partially insolubilized, impairing development, when the heating temperature is too high.
To overcome these problems, photosensitive compositions and planographic printing plate materials that have sufficiently high i.e., superior photosensitivity in the near-infrared region and do not require a heating treatment or an overcoat layer and allows image formation by scanning exposure are disclosed for example, in JP-A Nos. 2001-290271, 2002-278081, 2003-29408, and, 2003-43687. However, these planographic printing plate materials are still inadequate in developability and stability over time.